Guided-wave optical wavelength duplexers based on Mach-Zehnder interferometer constructions have been described in "Integrated four-channel mach-zehnder multiplexes fabricated with phosphorous doped SiO.sub.2 waveguides on Si", C.H. Henry et al, Topical Meeting On Integrated And Guided-Wave Optics, March 1988, Santa Fe, N. Mex., 1988 Technical Digest Series Volume 5 pp66-69. A typical configuration is shown in FIG. 1. The device has input ports 2, 4 to respective waveguides 6, 8 fabricated as elongate regions of increased refractive index in a crystal having a transparent surface. A directional coupler region 10 is provided in which the two waveguides 6, 8 are positioned sufficiently close together for light to be transferred from waveguide 8 to waveguide 6. Subsequently the two waveguides diverge and waveguide 8 extends in a straight line to a further directional coupler region 12 where the two waveguides are again positioned sufficiently close for light to be transferred between waveguides. The other waveguide 6 extends to region 12 in a curved contour having a longer path length (L+.DELTA.L) than the path length L of straight waveguide 8. Two output ports 14, 16 are provided. Thus in this arrangement incoming light at input port 4 containing spectral components a.sub.1 and a.sub.2 is split into two equal portions in region 10 and these portions are recombined in region 12. However, as a result of the incremental difference in path length light at wavelength a.sub.1 is combined constructively in waveguide 6 to provide an output at 14 comprising wavelength a.sub.1 whereas the light in waveguide 8 has the a.sub.1 component cancelled to provide an output at 16 comprising a.sub.2 wavelength. This effect is illustrated in FIG. 2 which shows the intensity of light output at output 14 as a function of wavelength. It can be seen the function is sinusoidal with a peak at a.sub.1 and a minimum at a.sub.2.
The path difference (.DELTA.L) introduces an optical phase difference -.DELTA.p where ##EQU1## a is the optical wavelength n(a) is the waveguide "effective" refractive index which is a function of (a) via the waveguide/material dispersion function.
If .DELTA.p is an even multiple of .pi. (180.degree.) all the light will exit at port 14. If .DELTA.p is an odd multiple of .pi. the light exits at 16. The output levels are sinusoidal functions of .DELTA.p and hence of a and .DELTA.L. It is desired to set .DELTA.L to such a value that the two design wavelengths exit at different ports with high purity.
In one envisaged application for the present invention, the duplexer is to be employed in an integrated optoelectronic transmit/receive chip which contains a laser transmitter and a photodetector receiver and a wavelength duplexer for separating incoming light (e.g. 1.53.mu.m wavelength) from outgoing light (e.g. 1.3.mu.m wavelength), the traffic in both directions being carried on the same optical fibre link which is coupled to the chip. A major problem with such an arrangement is that the signal being transmitted has a much higher intensity level than the received signal and therefore near-end crosstalk (reception of the outgoing signal by the photodetector) must be reduced as much as possible. In addition such a chip which will be manufactured in large numbers, should have its parameters correctly determined at manufacture without the need for subsequent tuning.
The known duplexer shown in FIG. 1 has some disadvantages for such an application. One problem is that curved portions of a waveguide which are employed to generate the differing path lengths tend to scatter radiation, even when fabricated with smooth edges. This problem is compounded in practice, since curved waveguides have a digitally stepped form arising from the mask-write step in the fabrication process. Thus the curved waveguide tends to be more lossy than the straight waveguide and this worsens crosstalk. In addition, the curved waveguide has transitional regions, in which losses occur, of changes in curvature, namely two straight line to curving transitions in the regions 18, 20 and curvature reversal regions 22, 24. As a further problem, mixing straight and curved waveguides makes the propagation parameters of the duplexer difficult to compute in advance.